1. Field of the Invention
This invention relates to rotary kiln plants for calcining and sintering mineral materials such as cement raw meal.
2. Background of the Invention
Rotary kiln plants of the type contemplated are advantageous whenever manufacturing plants requiring high outputs are concerned. The manufacturing process is thus arranged to be performed in separate units in a number of stages comprising a preheater, a calcining unit, a sintering unit in the form of a short rotary kiln and a cooler unit for cooling the hot kiln product.
The treatment of the raw material -- especially cement-forming raw materials such as cement raw meal -- in units divided in stages of treatment according to the various heat exchange steps in the process has been found attractive, primarily because economical production of the material can be obtained, but also because in such a plant it is possible to provide careful supervision and automatic control of the manufacturing process.
In the rotary kiln in plants of the type having a separate calcining unit, the amount of air necessary for combustion in the kiln is small so that only part of the heated cooling air may be effectively utilized in the rotary kiln for recovering the heat content of the cooling air. Another part is therefore utilized as combustion air in the calcining unit.
In prior art plants of this type, the cooler comprises either a number of cooler tubes arranged in planetary fashion around the outlet end of the rotary kiln rotating with the kiln so that all the heated cooling air to be used as combustion air is drawn through the rotary kiln. Alternatively, a grate cooler is combined with the rotary kiln in such a manner that at least part of the heated cooling air is utilized in the rotary kiln as secondary combustion air, whereas another part is utilized for preheating or combustion purposes in the calcining process performed in the calcining unit associated with the preheater.
However, with a separate planetary cooler or rotary cooler it has hitherto not been possible to combine an effective cooling of the hot product with satisfactory use of the amounts of heated cooling air as combustion air in the process, because it is difficult to construct and arrange the passage between the kiln and the cooler in such a manner that the heated cooling air and the hot product may pass in countercurrent to each other without encountering difficulties. A narrow passage (or passages) gives rise to dust problems and the cooling efficiency of the rotary cooler may be too low unless part of the cooling air is removed from the rotary cooler to overcome this problem. This results in a high overall heat comsumption of the process. Furthermore, although sufficient amounts of air are passed to the rotary kiln for use in the combustion process, the volume of air may be insufficient to cool the outlet of the kiln, the passage and/or the inlet of the cooler. Such insufficient cooling may result in damage being caused to either the passage or the cooler, or both due to the heat from the hot product. On the other hand, it is imperative that as much as possible of the heat content of the hot product is recovered. I have invented a plant which avoids these disadvantages while optimizing the transfer of heat to maintain substantial efficiency in the production of cement clinker.